The parting line here in the present invention in the mold-parting line, and at the same time, the very longest circumferential line which can be used as a mold-parting line dividing the orb into two equals; which line no dimple is allowed to intersect; the largest circumference line is one of the great circles dividing the orb in two equal hemispheres, and, unlike a symmetrical line that brings about an exact symmetry in each half, it does not necessarily bring about exact symmetries, but is such an axis as comes forth in a symmetrical position when one half of the orb is moved to a certain degree on a pole formed by one such axis; while the equivalent dimples are any such dimples as having shapes different from those of the ordinary dimples, being arranged in a single or double pairs in a symmetry beyond the mold-parting line, possessing the aerodynamic characteristics equal with or similar to the ordinary dimples, in replacement of those dimples that the mold-parting line intersects.
In production of a golf ball it is desirable to so design one as will ensure that the dimples are arranged in a symmetry with the parting line in the center so that the golfer can find either the parting line or the axis of symmetry with ease when placing the ball in a direction he desires for a tee-shot or putting, and it is also desirable to so arrange the dimples as to have the ball display the same aerodynamic characteristics regardless of which point the golfer strikes, because he is forbidden from readjusting the position of the ball, the only exception being the time of a tee-shot of putting.
With the view of optimizing a golf ball's non-directional aerodynamic characteristics attempts have been made by many to work out a dimple pattern to increase the number of axes of symmetry and largest circumference lines, and of all polyhedrons, a regular dodecahedron, icosahedron, or dodeca-icosahedron has been adopted in most cases to minimize the directional characteristics but maximize the number of axes of symmetry or largest circumference line.
But since a mold-parting line is indispensable in the process of production of a golf ball and since dimples can never be allowed on the mold-parting line, a dimple pattern has been extremely difficult to design by which dimples are evenly arranged, with plenty of axes of symmetry and a minimum of dimple-free areas provided on the surface of a golf ball.
In prior art, too, designs were made to arrange dimples on the basis of the axes of symmetry and the parting line in order to secure as many axes of symmetry as possible, but then, although aerodynamic characteristics improved, the dimple-free areas, too, increased because no dimple could be placed where axes of symmetry or the parting line intersected, and if dimples were rearranged on the parting line also in order to decrease dimple-free areas, the number of axes of symmetry decreased, resulting in a deterioration of the aerodynamic characteristics.
In the U.S. pat. No. 4,560,168, for example, it is apparent from the illustrations that although the 15 axes of symmetry and six great circles are all effectively exploited by way of arranging dimples making use of the six great circles quartering the spherical triangles of the icosahedron as a parting line, the dimple-free areas increase because it is necessary to arrange dimples in the equilateral triangles with the parting lines as their sides in a manner not to allow these dimples to intersect the parting lines.
Then in case these dimple-free areas are decreased, as in the U.S. Pat. No. 4,804,189, the number of axes of symmetry decreases to six or fewer, five of which having no directional traits at all, because it is necessary to adjust the locations and sizes of dimples alongside the parting lines in a manner where no dimple should intersect the mold-parting line; and so it becomes impossible to design a dimple pattern with non-directional aerodynamic characteristics.
In the U.S. Pat. No. 4,844,472, as another example, although it has as many as 15 axes of symmetry, which can be used as parting lines, on the basis of a regular icosahedron, it requires, as is apparent from the drawings, a distribution of exceedingly small dimples in those positions in which three of five axes of symmetry intersect, and despite all this there are left a number of dimple-free areas of fairly large size at the same points of intersection.
Now an objective of the present invention is to provide a golf ball which, with a dimple pattern to ensure an even and uniform arrangement of dimples on its surface designed by the use of a polyhedron of a certain form, is to have the largest number of axes of symmetry and largest circumference line according to the characteristics of the sides which constitute the polyhedron, disregarding whether or not dimples become intersected, but using as the mold-parting line one of the lines which, however, allow the least number of dimples to be intersected, and by eliminating only these least number of dimples that may become intersected, thereby minimizing dimple-free areas.
Another objective of the present invention is to provide a golf ball with the maximum non-directional symmetry by means of arranging a new equivalent dimples which have aerodynamic characteristics about equal to those of the ordinary dimples in the areas alongside the parting line whence the intersecting dimples are eliminated; that is, the dimple-free areas.